Left and right sided breast cancer.

Many studies have shown that unilateral breast cancer is more frequent in the left breast than in the right. This has been investigated in the Icelandic Cancer Registry. Information on all but 18 female breast cancer cases diagnosed in the forty-year-period from 1948 to 1987, a total of 2139 cases, was used. Of these 2011 were unilateral, 1069 were in the left breast, an excess of 13%. Primary breast cancer in both breasts was diagnosed in 81 women, 35 in the left breast first, and 46 in the right breast first. The excess risk of developing cancer remains for the left breast also for women who have already lost one breast because of cancer. Information on whether their relatives had developed breast cancer existed for 1197 of these women. Patients with an affected first degree relative were of 2.54 fold risk of developing contralateral primary breast cancer, but women with no affected relative were at a reduced risk (not significant). Patients with right sided breast cancer are more likely to have a relative with breast cancer. The breast cancer status of the relatives did not influence the risk of death, so a better survival of familial cases could not be shown.